1. Field of the invention
The present invention relates to a four-wheel steering vehicle adapted to steer rear wheels as well as front wheels, more particularly to a fail-safe system for a four-wheel steering vehicle.
2. Description of the Prior Art
In the steering system for four-wheel vehicles, recently, there has been proposed so called four-wheel steering vehicles as shown in Japanese patent public disclosure No. 60-163769 in which not only rear wheels but also front wheels are steered so that the drivability of the vehicles and the safety for the passengers can be improved.
In the four-wheel steering vehicle, the rear wheels are steered in the same direction as a direction in which the front wheels are steered (positive steering condition) in a high-vehicle speed operation. As a result, the vehicle is maintained substantially on a tangential line of a turning circle to obtain a smooth movement of the vehicle in such as a lane change operation in a high vehicle-speed condition. On the other hand, when the vehicle is under a low vehicle-speed operation, the rear wheels are steered in a direction opposite to a direction in which the front wheels are steered (negative steering condition) in a low vehicle-speed condition. As a result, it is easy to change the direction of the vehicle so that the turning radius is reduced to facilitate a U-turn operation, a parking operation, and a garaging operation.
There is disclosed in Japanese patent public disclosure No. 59-48054 a four-wheel steering system provided with a steering ratio changing mechanism in which a ratio of the steering amount of the rear wheels to that of the front wheels (steering ratio) is changed in accordance with the vehicle speed. The steering ratio changing mechanism is provided with a steering ratio sensor and a stepping motor, and a controller constituted by a micro computer wherein the controller carries out a feedback control of the stepping motor based on the output of the steering sensor in such a manner that the steering ratio changes continuously between a value of the positive steering condition and a value of the negative steering condition in accordance with the vehicle speed.
The four-wheel steering vehicles as disclosed in the Japanese patent public disclosure No. 59-48054 is provided with a rear wheel steering mechanism as well as a front wheel steering mechanism. The rear wheel steering mechanism is adapted to be driven by an assistance of a power steering mechanism actuated by a hydraulic pressure in relation with the steering ratio changing mechanism. The rear steering mechanism is provided with a solenoid valve for removing the hydraulic pressure applied to the power steering mechanism and a centering spring for returning the rear wheels back to a neutral direction thereof. In the case where a trouble occurs in the control system for the four-wheel steering system, the controller produces an emergency signal to actuate the solenoid valve and thereby release the hydraulic pressure applied to the power steering mechanism. As a result, the rear wheels is returned back to the neutral position by means of the centering spring so that the vehicle can be operated as well as a two-wheel steering vehicle and can obtain a safety in the four-wheel steering mechanism.
It should be however noted that if the hydraulic system for control the power steering system does not work properly, the safety of the vehicle would not be maintained.